This Old House--mistake with a shower stall heater?

Somehow found a tv show with people remodeling houses (there's a TV over my parents' house with a digital antenna; I don't have an antenna or cable).

Anyway the guy on This Old House did something really neat: he installed a shower stall with an in-floor radiator to keep the floor (and the exterior wall--one in there too) warm when showering.

He did something else really neat, too: he cut down a specially molded base for the shower stall, basically a piece of styrofoam molded such that it tapers to a drain hole in the center, so you lay something overtop and it supports the floor in a slope to the drain. Thinset that right down on top the radiator, then thinset a waterproof latex underlayment on top that, then installed the shower stall floor on top that.

... wait a minute.

Isn't 3 inch thick styrofoam a really good insulator?

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